1. Field of the Invention
The present invention relates generally to conversion assemblies for use on tilt rotor aircraft for converting from a helicopter mode to an airplane mode, and vice versa. In particular, the present invention relates to a method and apparatus for sensing the preload between a tilt rotor assembly and a wing when the tilt rotor aircraft is in the airplane mode.
2. Description of Related Art
Tilt rotor aircraft are hybrids between traditional helicopters and traditional propeller driven aircraft. Typical tilt rotor aircraft have fixed wings that terminate with convertible tilt rotor assemblies that house the engines and transmissions that drive the rotors. Tilt rotor aircraft are convertible from a helicopter mode, in which the tilt rotor aircraft can take-off, hover, and land like a helicopter; to an airplane mode, in which the tilt rotor aircraft can fly forward like a fixed-wing aircraft.
As one might expect, aside from the usual problems that must be addressed when designing either helicopters or propeller driven aircraft, the design of tilt rotor aircraft poses unique problems not associated with either helicopters or propeller driven aircraft. In particular, because the wings of tilt rotor aircraft must be designed to function in both the helicopter mode and the airplane mode, traditional design criteria used for helicopters or propeller driven aircraft alone are not sufficient. For example, the wings of tilt rotor aircraft often accommodate and support fuel tanks, interconnecting drive shafts from one engine to the other, interconnecting drive shafts from one conversion actuator to the other, redundant drive shafts, and spindles about which the tilt rotor assemblies and conversion actuators pivot. For these reasons, the space within the wings is extremely limited, resulting in little or no room for intrusive devices, measuring devices, sensing devices, or additional structural supports. Nevertheless, certain loads, both static and dynamic, must be carried by the wings of tilt rotor aircraft that are not present in either helicopters or fixed wing aircraft.
In a typical tilt rotor aircraft, the interconnecting drive shafts from one engine to the other are located near the trailing edges of the wings, as are the main spindles about which the tilt rotor assemblies pivot. Hydraulic conversion actuators for actuating the tilt rotor assemblies are pivotally carried at the wing tips and, in some instances, interconnected by shafts that run along the leading edges of the wings. This arrangement does not create problems when the tilt rotor aircraft is operating in the helicopter mode; but when the tilt rotor aircraft converts to the airplane mode, certain oscillatory vibration loads, such as longitudinal pitch loads and lateral yaw loads, are created by the rotors. Because of these unique airplane-mode loads, if a minimal structural stiffness is not maintained between the tilt rotor assembly and the wing, then the aircraft will become unstable. This minimal structural stiffness is based upon airplane-mode aircraft speed and related load factors. The internal preload of the conversion actuator increases the effective pitch stiffness of the tilt rotor assembly, but has little or no effect on the yaw stiffness of the tilt rotor assembly. To improve yaw stiffness, downstop assemblies with interlocking yaw restraints are used. However, the interlocking yaw restraints are only safe and effective if the tilt rotor assembly is forced against the wing so as to generate a preload sufficient to satisfy static and dynamic load requirements.
Certain attempts have been made to measure and maintain a selected preload between the tilt rotor assembly and the wing while the tilt rotor aircraft is in the airplane mode, but none have adequately resolved the problem. For example, in some tilt rotor aircraft, the preload between the tilt rotor assembly and the wing is measured using a complex closed loop algorithm that uses conversion actuator motor pressure to determine the preload between the tilt rotor assembly and the wing. In these applications, the preload between the tilt rotor assembly and the wing can be set, but with only limited accuracy. In other tilt rotor assemblies, an open loop system is employed in which the conversion actuators simply force the tilt rotor assembly into contact with the wing until the conversion actuator stalls. Such systems are undesirable in certain applications because allowing the preload to go to high requires added structural support resulting in increased weight and cost. In addition, these prior-art systems do not adequately compensate for the dynamic loads generated when the tilt rotor aircraft pulls up or goes into a dive.
Although great strides have been made in the design of tilt rotor aircraft, the problem of sensing and measuring the preload between a tilt rotor downstop assembly and a wing has not been adequately resolved.